I had never noticed a fritillary when we only had a purple (from HD, not sure of the variety). Now that we have a yellow, there always seem to be a couple in the garden. I would pick them off when the vines were smaller, but I'm not sure how much of dent they make now that the vines are large enough. I'll still pick off large ones within easy reach. I had also bought the BT but the vines are so vigorous I just never got around to it. I am about to bust it out on my younger citrus bushes though, as those seem to be swallowtail magnets, and definitely can't outpace the buggers.